According to statistics promulgated by the Energy Information Administration in 1990, there were approximately 35,000,000 residential water heaters operated by electricity out of a total of approximately 94,000,000 residential water heaters. The high cost of electricity relative to fuels such as natural gas is well known. On an operating cost basis, the cost per Btu of delivered water heating for electrical resistance water heaters is two to two and one-half times that of natural gas-fired water heaters. The concern for reducing energy consumption and conserving natural resources has given rise to a need for cost-effective substitutes for electrical resistance heating of water. Various substitutes for electrical resistance water heating that operate on electricity are known in the water heating industry. These substitutes include (a) solar-assisted water heating systems; (b) desuperheater water heating systems; (c) heat pump water heaters; and (d) fully integrated space conditioning/water heating heat pumps.
Solar-assisted water heating systems employ a bank of solar collectors to heat water with solar radiation. Well-designed solar systems can reduce the cost of heating water by about 50% as compared to electrical resistance heating. While effective from an energy usage standpoint, solar-assisted water heating has not achieved wide-spread usage because of the relatively high initial cost and maintenance costs, the potential for winter freeze-ups and unsightly appearance.
Desuperheater water heaters recover heat from hot compressor discharge vapor of a space conditioning system (which may be a heat pump). A refrigerant vapor-to-water heat exchanger (desuperheater) is inserted in the compressor discharge line of the space conditioning system. One disadvantage of desuperheaters is that when they are applied to a conventional air conditioning system, water heating is available from the desuperheater only when there is a simultaneous requirement for space cooling. If a desuperheater is applied to a heat pump, water heating availability is extended to times when there is a simultaneous requirement for space heating. Further, because a desuperheater is typically located outdoors adjacent to the space conditioning compressor, the water must be drained from the desuperheater during the winter months to prevent freeze-up. Alternatively, the desuperheater must be located remotely from the compressor in a conditioned space, which requires field installation of a set of hot refrigerant pipes between the desuperheater and the outdoor compressor discharge line. Because of the seasonal limitations, potential for winter freeze-ups, intrusion of the desuperheater into the space conditioning system refrigeration circuit and the high cost of field installation, desuperheaters are practical only in climates with extended space cooling requirements and very mild winters.
Dedicated heat pump water heaters heat water by extracting heat, typically from a conditioned space, and utilize heat pumping principles to transfer the heat to the water. In the typical application, indoor air is passed over an air-to-refrigerant heat exchanger that serves as an evaporator of a refrigeration system. The refrigerant vapor is then raised to a higher pressure by the compressor and then condensed in a refrigerant-to-water heat exchanger. During operation, the heat pump water heater generates a cooling effect on the indoor air passing over the air-to-refrigerant heat exchanger. This cooling effect is desirable when space cooling is required, but the cooling effect must be offset by additional space heating during the winter months. Further, in applications where air is drawn from and returned to the immediate indoor ambient environment, local cool spots may develop inside the structure. If the air-to-refrigerant heat exchanger is located outdoors, it is susceptible to frosting in winter. In addition to the problem of cool spots inside the structure or outdoor coil frosting, the dedicated heat pump water heater has a relatively high initial cost, which has limited its commercial acceptance.
Fully integrated space conditioning/water heating heat pumps provide both space conditioning and water heating. Such integrated systems also typically include a desuperheater, which is used for water heating when there is a simultaneous call for space conditioning and water heating. When there is no space conditioning demand, the system functions as a heat pump water heater. Integrated space conditioning/water heaters have not met with commercial success because of the complex piping and controls required and the relatively high initial cost.
An improved heat pump apparatus for heating liquids, such as water, is described in U.S. Pat. No. 5,305,614, assigned to Lennox Industries Inc. The heating apparatus includes two heat exchangers, one being a refrigerant-to-water heat exchanger external to a conventional hot water storage tank and the other being an air-to-refrigerant heat exchanger disposed in the return fluid stream of a systemically separate space conditioning system. Neither the space conditioning system nor the hot water storage tank need to be modified to accommodate the heat pump apparatus. In operation, heat is transferred from the return fluid stream to the refrigerant, which in turn transfers heat to the water, thereby providing "free" cooling of the return fluid stream and reducing the load on the space conditioning system when there is a demand for space cooling. In contrast to dedicated heat pump water heaters discussed hereinabove, the cooling by-product is not dumped into a conditioned space at one location, but rather is distributed throughout an indoor space by the space conditioning system ducting. One disadvantage of this type of heating apparatus is that when there is a demand for water heating, the indoor blower of the space conditioning system is activated to provide the return fluid stream. Because heating the water requires the return fluid stream to be cooled, cooled air is supplied to the indoor space, even when there is no demand for space cooling.
The present invention is directed to an improvement in the heat pump water heater described in U.S. Pat. No. 5,305,614.